The present invention relates generally to induction heating equipment and is particularly directed to a hand held induction tool of the type which produces a magnetic field in a predetermined direction. The invention is specifically disclosed as an induction heating tool that induces eddy currents in thin foil susceptors to raise the temperature of the susceptors to melt or soften an adhesive coating on the susceptor""s surface, for bonding trim to wallboard.
Historically, bonding materials quickly without use of mechanical fasteners has been quite difficult. Some of the problem areas which exist include: assembly time, cost of materials and labor, quality of the bond between component structures being assembled, reliability of the process of assembly, the typical requirement of fairly difficult finishing steps, convenience (or lack thereof) of use for the end-users, worker safety issues, and the difficulty in maintaining a good quality of appearance of the finished goods.
In the 1940""s, mechanical fasteners dominated the assembly industry, and adhesives were not as important to industry during this period. From the 1930""s through World War II, the United States and Germany began to develop plastics and adhesives technology in response to the growing scarcity of natural products. In many cases, particularly early on, adhesives have been used either in combination with mechanical fasteners or where no mechanical fastener could be effectively employed. Beginning in the 1950""s, the modern adhesives industry began to develop. Some of today""s more common adhesive systems, developed at that time, included heat-curable thermosets (epoxies), thermoplastic hot melts, pressure-sensitive adhesives (PSA""s), contact cements, water-based wood glues, and the super glues (cyanoacrylates). These were major disruptive technologies that have evolved over the last 45 years and which have slowly grown the fastening market and have significantly replaced traditional mechanical fasteners in many markets.
Adhesive bonding is generally superior to mechanical fastening, but present technology doesn""t allow for cost-effective pre-positioning and rapid development of a strong bond on demand with one step. Pre-positioning of components, prior to fastening, is very important, particularly in non-automated assembly systems. Millwork is an excellent example of such an assembly system. No current adhesive system allows for pre-positioning coupled with instantaneous bonding. Most of today""s adhesives are slow to cure, requiring minutes to hours, thus requiring clamping and other direct personal attention by the installer.
It should be noted that the ideal adhesive system is one where the adhesive cures on demand, is reversible on demand, has unlimited shelf life, has no VOC""s (volatile organic compounds), and is safe and easy to handle. Currently, the only such systems that exist are the light-curable systems, such as those employing UV (ultraviolet) and visible light. UV and visible light systems are unique in today""s adhesives world. They are liquid systems that cure only upon exposure to light. Optically transparent parts can be bonded in seconds or less to virtually any substrate. Such systems, where useful, have virtually replaced all other adhesive or mechanical fastening systems. An example would be automotive headlamp assemblies that do not need to be disassembled. It should be noted that UV-curable adhesives involve chemical handling and are not currently reversible.
There are two basic types of adhesive systems: one group of systems allows for pre-positioning of the parts to be bonded, yet by default, requires long cure times; the other group of systems provides very short, almost instantaneous cure times, but yet prevents pre-positioning of the parts.
Before describing some of the major adhesive systems available, one should be aware of the following general application notes that affect adhesive utility.
(1) Many product assembly sites are often dirty and difficult to keep clean. Certain adhesive systems cannot handle such situations.
(2) Temperature fluctuations at many assembly sites could be extreme, whether for an interior or exterior application. For example, a new home being built in the middle of the winter could see interior temperatures below 0xc2x0 F. Exterior applications could easily see temperatures even lower. Another typical example could involve automotive body repair, if done inside a non-heated building.
(3) Where humidity may be important, it is clear that the humidity around a manufacturing facility in Arizona would be far lower than that in and around a facility in Florida.
(4) The ability to directly heat many product components to cure adhesives is extremely limited, particularly as many plastic components can melt, and wood-based or cellulose-based millwork can bum.
One family of fast-curing adhesives is called xe2x80x9csuper gluesxe2x80x9d (cyanoacrylates). These adhesives allow for an extremely rapid adhesive setting, but cannot in any way be prepositioned before placement as the adhesive cures during positioning. Thus, there is no room for error. These adhesives are generally the most expensive adhesives. Furthermore, they are difficult to handle, and have a limited shelf life. Finally, there is no way to easily reverse cyanoacrylate, or super glue, bonds. Companies in this industry include Loctite Corporation, Henkel A.G., and National Starch.
Another instant adhesive technology, not often employed in structural applications, is pressure-sensitive adhesive (PSA) tapes. Like super glues, such products allow for extremely rapid adhesive bonding, but again, are extremely limited with regard to pre-positioning and, as with the xe2x80x9csuperglues,xe2x80x9d again, there is no room for error. Furthermore, pressure-sensitive adhesives are limited in their ultimate strengths unless they are thermosetting. In the case of a thermosetting PSA, some form of heat- or moisture-activation is required which is generally impractical for non-heat-resistant products, or where humidity controls are unavailable.
The latter two above thermosetting processes are time intensive. Even more importantly, pressure sensitive adhesives can be applied only in very narrow temperature ranges, typically from 55xc2x0 F. to 90xc2x0 F. Furthermore, above 90xc2x0 F., many common PSA""s weaken dramatically. As a further note on PSA""s, they are incapable of flow without heat to accommodate uneven surfaces, and if exposed to dust or other particulates, they immediately lose much of their potential adhesive strength. Finally, it is extremely difficult, if not impossible in most cases, to disassemble parts that use PSA""s. Examples of companies that manufacture PSA""s are 3M and Avery-Dennison, which are the two largest of the group. The cost of PSA""s can range from being some of the most inexpensive to some of the most expensive adhesives available today.
Hot melt adhesives are another example of an instantaneous or fast-cure system that significantly limits the ability to pre-position parts. Such adhesives are melted either in a large tank or in a small glue gun and are then dispensed as a molten material onto the parts. The parts. are then quickly mated, and the bond forms as the adhesive cools. The cooling process can be as short as a few seconds to possibly as long as ten or twenty seconds. As with the other instantaneous adhesives, there is little room for error, particularly where a clean and thin bond line is desired. Such limitations are the reasons that hot melt adhesives are used most extensively in the packaging industry and also for bonding small parts or surface areas. They are particularly useful in highly automated production systems, such as for sealing cereal boxes. Furthermore, such adhesives cannot be reheated after product assembly without significantly or entirely heating the product assembly.
On the positive side, hot melt adhesives are one-component, solid-state, zero VOC systems that have indefinite shelf life and, for the most part, are considered as plastics for regulatory and safety purposes. Furthermore, most hot melt adhesives are moderate to low in cost, especially when compared to the super glues or the light-curable adhesives. Examples of some leading hot melt manufacturers are Henkel A. G., Jowat, National Starch, H. B. Fuller, and Ato-Findley.
Other types of adhesive systems are those which are pre-positionable, but have long cure times. The most well known pre-positionable adhesives are the epoxies. Epoxy adhesives generally have slow cure times, usually on the order of minutes to hours, or even days. Most epoxies are two-part systems that, when mixed, become activated and cure. The catalysts are in one or both parts and their concentrations determine how quickly the epoxy adhesive will cure. In fact, if enough catalyst is added, epoxies can become instantaneous systems that are not pre-positionable. Epoxies are not difficult to handle, but do require special care as exposure can sometimes be detrimental to human health (causing skin irritations and burning).
Epoxies are among the strongest adhesives known, but require heat to achieve ultimate strength. A major problem with two-part epoxies is that cure time can vary dramatically with temperature. In fact, some systems cure so rapidly at temperatures above 90xc2x0 F. that they become almost unusable. At colder temperatures, e.g., below 60xc2x0 F., some systems may take days or more to cure. There also are one-component epoxies that cure only upon exposure to heat. Once heated, many one-component systems can cure in less than one minute. Epoxy bonds cannot be easily reversed. Examples of leading epoxy manufacturers include Ciba-Giegy, Shell Chemical, Henkel A. G., and Loctite.
Urethanes are another well-known, pre-positionable adhesive group. Like the epoxies, there are both two-part and one-part systems. After epoxies, urethanes are probably the second strongest class of commonly used adhesives. Two-part systems are the most common and generally take minutes to hours to cure. There are many one-part systems becoming available today which are moisture-curable (the moisture is actually a second part). Both systems have the problem that one component of the two, the isocyanate, is moisture-sensitive. If water gets into the adhesive, or if the humidity is too high, the isocyanate will react with the water, generate a gas, and cause foaming to occur. Even worse, if the moisture gets into a container unbeknownst to the user, and the container is then closed, the container can explode. As a result, two-component and moisture-cure urethanes are generally only used by skilled or specially trained personnel. Furthermore, because of their reactive nature and environmental susceptibility, most urethane adhesive systems require specialized mixing and dispensing equipment that must be cleaned on a frequent basis.
The primary advantage of most urethane adhesives is the availability of room temperature, moisture-curing, one-part systems that possess an overall lower application viscosity. This is as opposed to a two-part, room temperature epoxy that must be mixed, or a one-component hot melt that must be melted. Applications for urethane adhesives range from automotive assembly, to marine and aerospace assembly, to the millwork, furniture, and cabinetry industries.
It is important to note that certain adhesives have already been used with induction devices for many years. For example, such technologies are used for high strength bonds using relatively long cure-time (fifteen minutes to hours) adhesives. Furthermore, this technology generally employs high pressures to facilitate bond formation. This technology is used, for example, by Boeing, in the construction of composite-based passenger aircraft. The adhesive systems employed by Boeing are mainly epoxies. Such adhesives must be pre-positionable, and further must be cured over a long period of time because of the strict performance requirements mandated by the government for passenger and military aircraft.
Another company that employs similar technology is Emabond, a subsidiary of Ashland Chemical. Emabond develops the same types of long-time-cure adhesives (epoxies) as does Boeing, however, Emabond employs particulate susceptors which activate at higher frequencies that require operator shielding for safety. Emabond equipment is primarily geared toward automotive component assembly. A special piece of induction equipment is typically required for any two automotive components to be assembled.
Emabond employs a number of adhesive technologies, including epoxies, urethanes, and hot melt adhesives. Most of the adhesive systems used by Emabond are heat-activated by particulate susceptors, not foil susceptors, at higher frequencies that are known to be dangerous to human health (e.g., above 5 MHz). Moreover, the Emabond systems, primarily for the automotive industry, are part specific and are designed to bond generally irregular surfaces. The particulate susceptors allow for the use of liquid adhesives that can easily conform to these irregularities.
One method of bonding structures together utilizes susceptors made of an electrically conductive material that is heating by an alternating magnetic field to activate an adhesive material that resides on at least one surface of the susceptor. The magnetic field induces electrical currents, known as eddy currents, in the electrically conductive media. Exposure of such electrically conductive media to a magnetic field causes a temperature rise (heating) by what is termed the Joule effect. The Joule effect relates to heat generation due to the flow of electrons in a conductor. Distributions of these electrical currents and the heat they produce are not uniform in a conductive medium, such as a susceptor, exposed to an alternating magnetic field. The magnitude of heat, in Watts, is the sum of the heat contributions of all eddy current paths within the susceptor, each of which contributes heat that is equivalent to the product of its electrical resistance in Ohms and the square of its electron current in Amperes.
Within non-ferromagnetic susceptors, induced eddy currents have maximum intensities at the surfaces nearest the incident alternating magnetic field and have reduced intensities within the material, decreasing exponentially as a function of depth. This phenomenon is known as the skin effect, or the Kelvin effect, and the depth at which the eddy current falls to 37% is known as the depth of penetration. Most susceptors employed in the present application are comprised of a thin conductive sheet of uniform (or purposely non-uniform thickness) where, for the low frequencies usually used, the depth of penetration is far greater than the material thickness. Eddy currents at all depths within these susceptors are thus approximately equal, except where purposeful variations in susceptor thickness, or where open-space across the width of the susceptors cause variations in current density. In such cases, currents are forced to be non-uniform in specific regions to create more uniform heat generation or less uniform heat generation, depending on the specific application.
The magnitude of heat generated within a susceptor comprised of a conductive sheet of uniform thickness, is related to several factors. These factors include susceptor permeability, resistivity, size and shape, and the magnitude, frequency, size, and shape of the incident AC magnetic field. Variations of many of these parameters interrelate and affect the current distributions and densities that affect the sizes and locations of useful heat sources within the susceptors.
A Canadian patent by Krzeszowski, C A 1,110,961, (which is similar to U.S. Pat. No. 4,123,305) discloses a method for inductively heating a thermo-fusible material interposed between a carpet and a floor. An inductive heating tool is used to raise the temperature of a relatively thin-foil susceptor, which in turn activates the thermo-fusible adhesive material to create a bond, and thus xe2x80x9cgluexe2x80x9d the carpet to the floor. Krzeszowski discloses the use of a sheet of the thermo-fusible adhesive material, which is first placed upon the floor, followed by the carpet. Krzeszowski discloses the use of both continuously perforated sheets of aluminum as the susceptor material, or solid aluminum sheet. In one embodiment, a xe2x80x9cvapour-barrierxe2x80x9d sheet of aluminum (i.e., without perforations) is glued onto a slab of plaster, and then its other side is glued to a slab of expanded polystyrene, thereby creating a moisture barrier panel. One preferred aluminum sheet material disclosed in Krzeszowski is xe2x80x9cALBAL brand, reference 623,xe2x80x9d either with or without perforations.
The Boeing Aircraft Company owns several patents in the field of inductively heated susceptors. Virtually each patent extols the value of xe2x80x9ceven heatingxe2x80x9d of the susceptor to form a very high-strength and uniform bond. Of course, for aircraft structures, high strength bonds can be critical. Such patents include U.S. Pat. No. 3,996,402 (by Sindt), U.S. Pat. No. 5,717,191 (by Cristensen), U.S. Pat. No. 5,916,469 (by Scoles), and U.S. Pat. No. 5,500,511 (by Hansen). These patents use susceptors having various openings, and in some cases the openings are so large and numerous that the susceptor has an appearance of a screen-like material. All of the susceptors specified by the above Boeing patents have thickness dimensions that exceed 0.003 inches (3 mils). Such devices are not particularly useful in xe2x80x9cquickxe2x80x9d bonding of substrates.
Previous induction heating devices suffer from an inability to be made truly portable, i.e., lightweight, while simultaneously delivering the energy necessary to form bonds in short periods of time. It would be desirable, especially for higher-speed, lower-strength bonding applications, to provide an induction adhesive activation device with corresponding susceptor design that accumulates the heat in the susceptor and the adhesive while simultaneously withholding significant conduction losses to the substrates until all of the adhesive had either melted, begun chemical reaction, flowed adequately, or all three occurred.
Such a system would be valuable if the bonds developed were as strong as typically required for as wide a range of applications as possible, and it would be even more valuable if the susceptor adhesive device were optionally reversible by design. Such an induction adhesive activation device would ideally have improved energy efficiencies, sufficient to enable operation with a battery, be lightweight, support high duty-cycle operation ( greater than 40%) for many hours at a time, and require no liquid cooling.
It would be advantageous to provide an induction-based adhesive technology that can bond nearly instantaneously on demand, and which is not directed toward a pre-positionable adhesive, thereby allowing for simplified, more rapid production, and eliminating the requirement of high-energy systems such as those that operate at high frequencies that are known to be dangerous to human health.
Accordingly, it is an advantage of the present invention to provide an inductive heating apparatus having a work coil that is driven by an electrical current, in which an electrical current at a power converter stage is increased to a predetermined magnitude, whereupon the voltage magnitude is measured and used to determine a xe2x80x9cdistance valuexe2x80x9d that substantially represents the actual distance between the work coil and a susceptor, then based upon that distance value, the current and voltage magnitudes are adjusted so as to cause said work coil to emit a magnetic field at a predetermined desired power level that will induce eddy currents in the susceptor""s electrical conductor to cause a temperature rise.
It is another advantage of the present invention to provide an inductive heating apparatus having a work coil that is driven by an electrical current, in which both a current magnitude and a voltage magnitude of a power converter stage are sampled over multiple sampling time intervals, and the average current and voltage magnitudes are used to calculate a power level being generated, and the power level is then adjusted according to a profiled energy delivery scheme until achieving a predetermined accumulated energy for the profiled energy delivery scheme, which ends the power cycle.
It is a further advantage of the present invention to provide a susceptor apparatus consisting of a strip-like structure having a length, width, and thickness, in which the structure includes at least one layer of an electrically conductive material and at least one layer of an adhesive material proximal to at least one of its outer surfaces, wherein the electrically conductive material generates an eddy current when exposed to a magnetic field of a predetermined minimum intensity, and the layer of electrically conductive material exhibits a thickness in the range of 0.10 through 3 mils.
It is yet another advantage of the present invention to provide a susceptor apparatus consisting of a strip-like structure having a length, width, and thickness, in which the structure includes at least one layer of an electrically conductive material and at least one layer of an adhesive material proximal to at least one of its outer surfaces, wherein the electrically conductive material generates an eddy current when exposed to a magnetic field of a predetermined minimum intensity, and the at least one layer of an electrically conductive material comprises at least two individual layers of electrically conductive material, in which each of the electrically conductive layers is separated from one another by an insulative layer.
It is still another advantage of the present invention to provide a susceptor apparatus consisting of a strip-like structure having a length, width, and thickness, in which the structure includes at least one layer of an electrically conductive material and at least one layer of an adhesive material proximal to at least one of its outer surfaces, wherein the electrically conductive material generates an eddy current when exposed to a magnetic field of a predetermined minimum intensity, and the structure contains at least one fusible portion that melts more quickly than other portions of said structure when its temperature is raised.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a method for controlling energy delivered by a work coil comprises: providing a heating apparatus having a work coil, and an electrical circuit that provides an electrical current to the work coil; emitting a magnetic field from the work coil, increasing a first electrical characteristic of the electrical circuit to a predetermined first magnitude, and substantially at the predetermined first magnitude, determining a second magnitude of a second electrical characteristic of the electrical circuit to determine a distance value; and based substantially upon the distance value, adjusting the first electrical characteristic and the second electrical characteristic to emit the magnetic field at a predetermined desired power level useful at the distance value.
In accordance with another aspect of the present invention, a method for controlling energy dissipated in a susceptor being delivered by a work coil comprises: providing a heating apparatus having a work coil, and an electrical circuit including a power converter; emitting a magnetic field from the work coil, and directing the magnetic field toward a susceptor that contains an electrically conductive portion that produces eddy currents due to the magnetic field; adjusting a first electrical characteristic of the power converter to a first predetermined magnitude, determining a second electrical characteristic of the power converter substantially at the first predetermined magnitude, then determining a distance value substantially corresponding to a physical distance between the work coil and the susceptor; and based upon the distance value, automatically adjusting the first and second electrical characteristics of the power converter to thereby emit the magnetic field at a power level sufficient to raise a temperature of the electrically conductive portion of the susceptor.
In accordance with yet another aspect of the present invention, a heating apparatus is provided, comprising: an electronic circuit including a power converter stage, and a work coil, the electronic circuit being configured to control a current magnitude and a voltage magnitude at an input of the power converter stage; the electronic circuit being configured to adjust one of the current magnitude or voltage magnitude at the input of the power converter stage to a first predetermined magnitude, and to use the other magnitude to determine a distance value; and based upon the distance value, the electronic circuit is further configured to adjust the current magnitude and voltage magnitude at the input to the power converter stage so as to cause the work coil to emit a magnetic field at a predetermined desired power level.
In accordance with still another aspect of the present invention, a heating apparatus is provided, comprising: a work coil and an electronic circuit, the electronic circuit, upon actuation of a cycle, being configured to determine both a current magnitude and a voltage magnitude input to a power converter stage over a plurality of sampled time intervals, then being configured to average the current and voltage magnitudes to calculate a power level being consumed by the power converter stage; and the electronic circuit being further configured to adjust the power level according to a profiled energy delivery scheme until achieving a predetermined accumulated energy for the profiled energy delivery scheme, and terminating the heating cycle.
In accordance with a further aspect of the present invention, a susceptor apparatus is provided, comprising: a substantially thin structure having a length, width, and thickness, the structure including at least one layer of an electrically conductive material, the structure including at least one layer of an adhesive material proximal to at least one of its outer surfaces, and in which the at least one layer of an electrically conductive material comprises a first layer of electrically conductive material, a second layer of electrically conductive material, and a layer of electrically insulative material positioned therebetween; and the structure being of a characteristic by which the at least one layer of electrically conductive material generates an eddy current when exposed to a magnetic field of a predetermined minimum intensity.
In accordance with yet a further aspect of the present invention, a susceptor apparatus is provided, comprising: a strip-like structure having a length, width, and thickness, and having at least one edge, the structure including at least one layer of an electrically conductive material, the structure including at least one layer of an adhesive material proximal to at least one of its outer surfaces; and the structure containing at least one fusible portion that melts more quickly than other portions of the structure when its temperature is raised.
In accordance with still a further aspect of the present invention, dn apparatus is provided having at least one structure made of a magnetic material having a predetermined shape to create at least two magnetic poles at times when a magnetic field flows therethrough; an electrically conductive winding that is wrapped around at least one of the magnetic poles; and the structure is sub-divided into at least two portions, a first of portion having a substantially curved end of a concave shape, and a second portion having a substantially curved end of a convex shape that substantially mates against the concave curved end.
In accordance with still another aspect of the present invention, a method of adhesive bonding by induction heating includes: providing a susceptor structure having a length, width, and thickness, the structure including at least one layer of an electrically conductive material and including at least one layer of an adhesive material proximal to at least one of its outer surfaces, the structure being of a first characteristic by which said at least one layer of electrically conductive material generates an eddy current when exposed to a magnetic field of a predetermined minimum intensity, and the structure being of a second characteristic by which the at least one layer of electrically conductive material exhibits a thickness in the range of 0.01 mils (0.25 microns) through 3 mils (76 microns); and exposing the susceptor structure to a magnetic field during a heating event for a time interval in the range of 0.05-10 seconds, inclusive, and at an average power density in the range of 10-5000 Watts per square inch, inclusive.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.